I think you should read some documentation. However your "req" object is file-like, so you can use read() method to get its contents.
def fileDownload(self, some_Id, range=None):
# [...] something happens here
# get the Amazon URL
fileUrl = we_get_the_amazon_url_here
req = urllib2.urlopen(fileUrl)
if len(range):
req.headers['Range']='bytes=%s-%s' % (range[0], range[1])
return req.read()